There are many manufacturing facilities in the United States which utilize production tunnel kilns of the type comprising a preheat section and a firing section. These production tunnel kilns have a conveyor belt which passes longitudinally through the kiln. Carried on the conveyor belt are a plurality of boards. On the boards are substrates having a paint screened thereon and the boards are carried into the preheat portion of the kiln wherein the temperature is raised to approximately 600 degrees. The preheat section of the kiln removes volatile organic material that is emitted or escapes from the paints. The boards and substrates then pass into the firing section of the kiln where the firing temperature is approximately 800.degree. C. or higher and the materials are cured. In the prior art, there have been considerable problems with cross-contamination which occurs when a board has substrates with a paint different than the paint on substrates on preceding boards. In other words, boards with "wet" paints first pass through the preheat section of the kiln and are raised to a temperature which removes volatile organics from the paints. Volatile organics remaining in the atmosphere can contaminate different paints which subsequently appear on successive boards. In the prior art, this cross-contamination of different types of paints has prompted attempts to vent or remove the contaminated atmosphere from the preheat section of the kiln. This has been done in generally one of two ways, either by forcing a co-flow or counter-flow of purging air longitudinally throughout the preheat section of the kiln. A co-flow venting system forces air longitudinally through the oven in the same direction as the conveyor belt moves, and a counter-flow venting system passes air counter to the direction of movement of the conveyor belt. Both systems attempt to create a turbulence in the air, and attempt to eliminate laminar air flow in order that the volatile organics can be vented at one end or the other of the preheat section of the kiln, depending on the type of venting system being used. Such venting systems have proved ineffective because turbulent air flow is not created throughout the length of the kiln. Turbulence generally does not occur in the prior proposals throughout the center portion of the kiln section. There frequently remains a laminar type air flow at the center kiln section. This laminar flow at the center section of the preheat oven permits the volatile organics to remain in the kiln atmosphere where they come into contact with subsequent boards having different paints passing through the kiln.
Another method that has been attempted in order to eliminate contaminated atmosphere from the preheat section of the kiln, is to have a number of consecutive boards with no substrates thereon so that there will be a gap between the different types of paints on substrates passing through the kiln. This has been done by having as many as 20 empty boards pass through the preheat section before a new group of substrates having a different paint enter the preheat section. Obviously, this has a great effect upon the yield capacity of a kiln and is highly unsatisfactory for economical production methods. Therefore, it would be desirable to provide an atmospheric purging system for the kiln interior which would effectively remove volatile organics contaminating the atmosphere, and which would improve the yield capacity of a kiln by allowing different paints to pass through the kiln without contamination occurring and with a maximum number of painted substrates passing through the oven during its operational period.
Another problem created by kilns which do not effectively vent contaminated atmosphere, is the inability to track effectively the temperature coefficient of resistance (TCR) of fired resistors in order that the resistors meet customer specifications. Manufacturers must be able to produce fired resistors that meet specifications requiring not only a particular fired resistance value and an overall TCR range but which also require a specified TCR tracking value (TCRTV). The TCR tracking value (TCRTV) may be defined as the difference between the changed resistance values of resistors as the ambient and operational temperatures of each resistor varies during operational use, such a change having a grossly distorting effect on the circuitry. It is highly desirable that as the temperature of the resistors change during operation, and the respective resistance values change, the resistance values change at approximately the same rate so that the resulting differential between the changed resistance values and thus the output characteristics of the resistors or network of resistors remain as consistent or controlled as possible. The match of initial fired resistance values must be viewed as important not only from a static viewpoint but from a dynamic viewpoint. A different rate of change in resistance values of the respective resistors or networks can alter undesirably the differential (TCRTV) between the respective changed resistance values and thus the overall output characteristics.
The TCR of a fired resistor is dependent upon or reflects the fired resistance value of the resistor, and correspondingly, the TCRTV also reflects the fired resistance value of the resistor. Thus, if resistors have a divergence of resistance values after sintering in the firing section of the kiln, the TCR and TCR tracking values of the resistors tends to have a corresponding divergence and the customers specifications are not obtained. Thus, the match of initial fired resistor values is important, but equally important is a continued match during change in ambient and operational temperatures so that this match is a dynamic match remaining equivalent at all equilibrium temperatures as well as at initial temperatures. If there is contamination, there tends to be a disfunction of match thereby making an effective kiln exhaust system all the more important. Therefore, it is desirable to increase the quality of resistors produced by a tunnel kiln firing procedure by improving the consistency of fired resistance values obtained, thereby lending to an improved consistency of TCR values which will tend to result in a smaller differential in changed fired resistance values and lead to an improved TCRTV.